Method and apparatus for using Doppler modulation parameters for estimation of vibration amplitude

ABSTRACT

A system for using Doppler modulation parameters for producing an estimation of the vibration amplitude of an object under investigation is disclosed in which a low frequency vibration source is used to force the oscillation of the object under investigation and a coherent or pulsed ultrasound imaging system is utilized to detect the spatial distribution of the vibration amplitude of the object in real-time. The reflected Doppler shifted waveform generated by reflecting the ultrasound waves off of the vibrating object under investigation is used to compute the vibration amplitude and frequency of the object using two alternative methods, either on a frequency domain estimator basis or on a time domain estimator basis.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.07/608,391, filed concurrently herewith, and entitled METHOD ANDAPPARATUS FOR BREAST IMAGING AND TUMOR DETECTION USING MODAL VIBRATIONANALYSIS, which is commonly assigned.

BACKGROUND OF THE INVENTION

The present invention relates generally to the use of low frequencyvibrations to investigate passive objects. More particularly, thepresent invention relates to a method of and a system for using lowfrequency external stimulation to vibrate an object and then using asecond high frequency wave energy reflected by an object for analyzingthe vibration of the object created by the generated low frequencyvibrations in order to examine the surface or cross-section of theobject and to produce an image of the examined object.

It is frequently useful to examine passive structures or objects bymeans of the application of a swept frequency vibration or audio source.Structures which are typically examined in that manner for flaws includeaircraft, ships, bridge trusses and other types of large structures. Inaddition, soft tissue can also be examined in that manner.

In the examination of such structures, the vibration sources may betemporarily mounted on exterior surfaces of, for example, ship hulls orbridge trusses, or may be placed in the interior of the structures to beexamined, such as ship hulls or passenger aircraft. The interrogatingwave energy is then focused on a reference point on or within theobject.

Such interrogating wave energy may vary from a laser, microwave orairborne ultrasound source, for use with aircraft, bridges, ships orother structures. Sonar or ultrasound sources may be utilized forunderwater inspection. The vibration source of low frequency is sweptover a broad frequency range so as to excite eigenmodes with relativelyhigh vibrational amplitudes. Once an appropriate vibration frequency isfound, the interrogating wave energy can then be scanned over thesurface or within a cross-section of the interior for penetrating waves.The spot size or spatial resolution of the scan, depends upon thewavelength of the vibration source used and the particular apparatusused. For simply focused coherent sources, the spot size will be equalto (1.2)(wavelength) (focal length)/(aperture radius).

Since sub-millimeter wavelengths can be achieved by the instantinvention using ultrasound and optical devices as interrogating wavesources, millimeter scale, spatial resolution can be achieved utilizingthe present invention. Once an appropriate scan size or region ofinterest is selected, an image is derived from point-by-pointexamination of the Doppler shift of the reflection of the interrogatingsource back from the object. Although different prior techniques havebeen described which analyze vibrations using lasers or ultrasound, noneof those techniques make use of externally applied vibration andpoint-by-point scanning using the method of the present invention, togenerate a vibration image.

The method of the present invention is useful to generate an image whoseintensity or color is proportional to the vibration amplitude calculatedby means of the inventive method, at each point on the object. Thatimage can be inspected for modal shapes and abnormally high or lowvibration amplitudes, and can also be compared with reference imagesobtained earlier or from well characterized analogous structures. Thetime required for analysis of each spot utilizing the inventive methodand system described herein is less than three cycles of the vibrationfrequency when the frequency domain estimator method is utilized and afraction of a single vibration cycle if the time domain estimator methodis utilized. Thus, those methods can be applied rapidly so as to permitreal-time imaging. Because the methods are also sensitive to vibrationbut are stable in the presence of noise, vibrational amplitudes of lessthan 1/10 of a wavelength of the interrogating wave energy can be easilydetected utilizing the inventive method and system.

One prior art approach to measuring amplitudes of vibration is shown inU.S. Pat. No. 4,819,649, issued Apr. 11, 1989, to Rogers et al. Thatpatent is directed to a non-invasive vibration measurement system andmethod for measuring the acoustically induced vibrations within a livingorganism. That patent utilizes a continuous wave of high spectral purityultrasonic beams and utilizes two separate transducers, one fortransmitting and one for receiving the focused beams.

By virtue of its frequency domain processing, the device disclosed byRogers et al. cannot produce real-time imaging. In fact, the '649 patentdoes not discuss imaging at all. All of the specific implementationsdiscussed in the '649 patent relate to frequency domain techniques whichare based upon the ratio of harmonic sidebands of the reflected signal,which allows the intrusion of noise elements into the reflected sampleand, thus, into any analyzed signal.

The system and method disclosed in the '649 patent is also discussed inan article written by the inventors which appeared in the Journal ofVibration, Acoustics, Stress and Reliability in Design, entitled"Automated Non-Invasive Motion Measurement of Auditory Organs in FishUsing Ultrasound", Vol. 109, January 1987, pp. 55-59. That paperdiscloses the use of external vibration to produce an FM Doppler shiftand examines, over long periods of time, the Doppler spectrum returningfrom a single point. It is not a real-time system. The article assumesthat, using very small vibrations, the ratio of the carrier signal tothe first harmonic is indicative of the vibration amplitude. Like the'649 patent, the device of Rogers and Cox disclosed in this paper is anon-scanning, slow, frequency domain method which uses the ratio ofharmonic sidebands and is restricted to use with very low amplitudes.

Another approach used in the past is disclosed in an article entitled"Imaging the Amplitude of Vibration Inside the Soft Tissues for ForcedLow Frequency Vibration", by Yamakoshi, Mori and Sato, published in theJapanese Journal of Medical Ultrasonics, Vol. 16, No. 3, pp. 221-229(1989). That article discusses an imaging system which can observe theprecise movements inside of soft tissues when an external vibration isapplied to those tissues. While the system described in that articledoes scan and make vibration images, it uses frequency domain techniqueswhich are also based on the ratio of harmonic sidebands approach whichare noise sensitive. Furthermore, it is slow and not a practicalreal-time system and is restricted to use with small amplitudes ofvibration.

Yet another approach used in the prior art is that of Pierce andBerthelot, as disclosed in Proceedings of the SPIE, Session EE.Engineering Acoustics IV: "Laboratory and Measurement Microphone","Absolute Calibration of Acoustic Sensors Utilizing ElectromagneticScattering from In Situ Particulate Matter", Pierce and Yves (1988).That paper describes the same FM Doppler spectrum utilized by Cox andRogers and as described in their article discussed above. However,Pierce et al. utilize laser methods to measure the oscillation at apoint. The methodology of Pierce et al. is a non-scanning, slow,frequency domain technique which again uses the ratio of harmonicsidebands and therefore suffers from the same noise sensitivity problemsas do the other prior systems discussed above.

All of the known techniques can be broadly classified as utilizing thesame approach to the estimation or determination of the vibrationalparameters, that is, using some ratio of spectral harmonic amplitudes.Thus, they all suffer from the disadvantages of the ratio methodsbecause they require either intensive computation or larger lookuptables of theoretical Bessel functions for comparison with the measureddata. Further, ratio methods work well only when the argument of theBessel function is small, which poses a severe limitation on the rangeof the estimation of the Doppler spectrum.

As a practical matter, the performance of the ratio methods is highlydegraded since almost all Doppler spectra suffer from a poorsignal-to-noise ratio. Additionally, a sophisticated algorithm isrequired to determine the best selection of the harmonic pair to becompared. The present invention, on the other hand, utilizes a simpleand noise-immune method for vibration estimation or determination.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the foregoing, it should be apparent that there still existsa need in the art for a method of and system for providing for theremote inspection of structures or soft tissues using an externallyapplied vibration source and which provides for the generation inreal-time of an analysis of the returned signal from the object underinvestigation for Doppler shift properties. It is, therefore, a primaryobject of this invention to provide a method of and system for remotelyinspecting structures and soft tissues using externally appliedvibration and Doppler shift measurement which is characterized byproviding an analysis on a real-time basis of the return signal of aninterrogating wave for Doppler shift properties.

More particularly, it is an object of this invention to provide a remoteinspection system as aforementioned which provides for an analysis ofthe Doppler shifted interrogation signal in order to derive the localvibration amplitude of the reflector or object under investigation aseither a gray scale or color image.

Still more particularly, it is an object of this invention to provide asystem for remotely inspecting objects in which the image whichrepresents the analyzed Doppler shifted signal reflected from the objectis generated by scanning the object with wave energy such as laserbeams, microwaves, sonar, or ultrasound.

Another object of the present invention is to provide a system for theremote inspection of vibrating structures which is not restricted tosmall amplitudes and is noise insensitive.

A further object of the present invention is to provide a system for theremote inspection of objects which utilizes a time domain method whichpermits the real-time imaging of the vibration amplitudes produced byreflection of the interrogating wave from the object over a region ofinterest from as little as three samples of quadrature components in thetime domain.

A still further object of the present invention is to provide a systemfor the remote inspection of objects in which a time domain estimatormethod is utilized but which provides a system which is not restrictedto small vibration amplitudes and is not a ratio of harmonic sidebandsestimators. Thus, the system is very flexible and noise insensitive.

Briefly described, these and other objects of the invention areaccomplished in accordance with its system aspects by using a lowfrequency vibration source to force the oscillation of an object underinvestigation and a pulsed ultrasound imaging system is utilized todetect the spatial distribution of vibration amplitude of the object inreal-time. Due to the different mechanical properties of the abnormalregions of the object from those of normal regions, vibration patternsof the object at different vibration frequencies are used to determinethe abnormalities. A vibration source generates a source of vibrationswhich is fed to a pulsed or coherent ultrasound imaging device as wellas being transmitted onto the object under investigation.

The reflected Doppler shifted waveform generated by reflecting thepulsed ultrasound waves off of the vibrating object under investigationis received by the pulsed ultrasound imaging device which computes thevibration amplitude and frequency of the object in order to form avibration image. The computations use two alternative methods, either ona frequency domain estimator basis or on a time domain estimator basis.Those methods may or may not include a noise removal process, dependingupon the working environment.

The results of calculations of vibration amplitude and phase from sometarget position are stored in a conventional scan converter whichproduces an image of the estimates as a function of position over theentire scan plane. Also, synchronization between the estimator systemsand the vibration source may or may not be employed, depending upon theactual vibration estimation method utilized.

With these and other objects, advantages and features of the inventionthat may become hereinafter apparent, the nature of the invention may bemore clearly understood by reference to the following detaileddescription of the invention, the appended claims and to the severaldrawings attached herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the system of the present invention;

FIG. 2 is a block diagram showing the apparatus of the presentinvention;

FIG. 3 is a block diagram showing the system for noise correction forthe frequency domain estimation method embodiment of the presentinvention;

FIG. 4 is a block diagram showing the system for the time domainestimation method embodiment of the present invention;

FIG. 5 is a schematic block diagram showing the phase and co-phasesignal estimation system for the time domain estimation system of FIG.4;

FIG. 6 is schematic block diagram of additional phase processingcircuitry which may be utilized in connection with the time domainprocessing circuitry of FIG. 4;

FIG. 7 is a schematic block diagram of successivephase estimatorcircuitry which may be utilized in connection with the phase andco-phase estimation circuitry of FIG. 4;

FIG. 8 is a schematic block diagram of successive co-phase estimatorcircuitry which may be utilized in connection with the phase andco-phase estimation circuitry of FIG. 4;

FIG. 9 is a schematic block diagram of bi-phase estimator circuitrywhich may be utilized with the phase and co-phase estimation circuitryof FIG. 4;

FIG. 10 is schematic block diagram of 1-shift cross-phase estimatorcircuitry which may be utilized with the phase and co-phase estimationcircuitry of FIG. 4;

FIG. 11 is schematic block diagram of the phase-ratio estimatorcircuitry for vibration frequency estimation for use as part of thecircuitry of FIG. 4;

FIG. 12 is a schematic block diagram of the autocorrelation estimatorcircuitry which may be utilized in conjunction with the circuitry ofFIG. 4;

FIG. 13A is a schematic block diagram illustrating how a conventionalvelocity estimator can be utilized to provide an improved approximateestimation of vibration amplitude signal; and

FIG. 13B is a schematic block diagram showing how a conventionalvariance estimator can be utilized to produce an improved approximateestimation of vibration amplitude signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to detail in the drawings wherein like parts aredesignated by like reference numerals throughout, there is illustratedin FIG. 1 in block diagram form the overall system of the presentinvention. A low frequency (approximately 1-1,000 Hertz) vibrationsource 100 applies a signal to the object under investigation 102 inorder to cause the object under investigation 102 to begin vibrating.

An ultrasound transmitter and receiver 110 is used to generateultrasonic radiation which is aimed at a point on or in the object underinvestigation. The waves reflected from the object under investigationare received by the receiver portion of the ultrasound transceiver 110and supplied as an input to the pulse ultrasound imaging device 104. Thevibration source 100 is connected to supply the same signal which isused to vibrate the object under investigation 102 to the pulseultrasound imaging device 104. The pulse ultrasound imaging device 104may be an ACUSON 128 Color Doppler Imager available from ACUSON,Mountain View, Calif.

The pulse ultrasound imaging device 104 receives the vibration signalgenerated by the vibration source 100 as well as the ultrasoundreflected signals from the object under investigation and utilizes thetwo signals to produce the received signals 106. The received signalsare input into a vibration parameter calculator 108, which maypreferably be specialized real-time circuits as shown in FIGS. 3-12.Alternatively, it may be a computer, such as a Sun 3/50 workstationnetworked to a Sun disk server. Alternatively, a personal computerhaving real-time digital signal processing characteristics could beutilized in place of the Sun computer.

The vibration parameter calculator 108 generates the vibration images112 which may be displayed by means of a color or monochrome CRT, laseror other printer or other display device 113.

The pulse ultrasound imaging device 104 is used to detect the spatialdistribution of vibration amplitude in real-time. The spatialdistribution of the vibration amplitude is generated by means of thevibration source 100, which may preferably be a 5" diameter wooferloudspeaker available from the Radio Shack Division of TandyCorporation, Dallas, Tex., and others, causing the object underinvestigation 102 to vibrate, and the ultrasound transceiver 110, whichmay be a 3.5 MHz or 5 MHz array available from ACUSON.

Due to the different mechanical properties of the abnormal regions ofthe object under investigation 102 from those of its normal regions,vibration patterns of the object 102 at different vibration frequenciesare used to reveal the abnormalities.

It should be understood that the vibration source 100 may use sound orany type of electromagnetic radiation which causes the object underinvestigation to vibrate. In addition, the ultrasound transmitter andreceiver 110 may alternatively utilize any type of coherent, pulsed orcontinuous electromagnetic radiation that can be detected by theappropriate imaging device 104, for example, light waves, microwaves,etc. It is, however, necessary that both the vibration source 100 andthe transmitter 110 utilize coherent radiation which can be readilydetected by the imaging device 104.

As disclosed in FIG. 1, the present invention provides for the remoteanalysis and imaging using, for example, radar, microwaves, sonar,ultrasound, lasers, or any other type of electromagnetic radiation ofvibrated structures such as airplanes, bridges, ship hulls or tissue, todetect flaws and cracks as revealed by the abnormal vibration amplitudesgenerated by such flaws. The stress concentrations which occur aroundsuch cracks and flaws are detected by using the Doppler techniquesdescribed herein.

The image is generated by scanning the vibrating object underinvestigation 102 with coherent, pulsed or continuous, focused orunfocussed wave energy such as laser, microwaves, sonar or ultrasound,and analyzing the return signal for Doppler shift properties. By usingeither the frequency domain estimation method shown and described inconnection with FIG. 3 herein or the time domain estimation method shownand described in connection with FIGS. 4-12 herein, the Doppler shiftedsignal is analyzed 106 to derive the local vibration amplitude of thereflector or object under investigation 102. The result, when combinedwith information regarding the position of the interrogated object 114is vibration images 112, which can be displayed as either a gray scaleor a color image.

In general, the method of the present invention for the examination of apassive structure requires the application of a swept frequencyvibration or audio source, such as the vibration source 100. Inexamining a passenger aircraft, for example, that may be accomplished byplacing a bank of loud speakers in the interior. Vibration sources mayalso be temporarily mounted on exterior surfaces, such as the wings,hulls of a ship or bridge trusses. The interrogating wave energygenerated, for example, by the ultrasound transceiver 110, is thenfocused on a reference point or within the object. The vibration source100 is swept over a broad frequency range so as to excite eigenmodes ofthe object under investigation 102 with relatively high vibrationalamplitudes.

Upon finding an appropriate vibration frequency, the interrogating waveenergy is scanned over the surface or within a cross-section of theinterior of the object under investigation 102 for penetrating waves.The time required for analysis of each spot is less than three cycles ofthe vibration frequency when the vibration parameter calculator 108 isutilizing the disclosed frequency domain estimator method and a fractionof a single vibration cycle where the vibration parameter calculator 108is utilizing the time domain estimator method disclosed herein.

The spot size, or spatial resolution of the scan produced by thetransceiver 110, depends upon the wavelength used and the particularapparatus. For simply focused coherent sources, it has been found thatthe spot size will be equal to (1.2)(wavelength)(focal length)/(aperture radius). Since the present invention achieves the use ofsub-millimeter wavelengths using ultrasound and optical devices,millimeter scale spatial resolution is achieved. Also, vibrationalamplitudes of less than 1/10th of one wavelength of interrogating waveenergy can be easily detected utilizing the estimation methods in thevibration parameter calculator 108.

Upon selecting a scan size or region of interest, an image is derivedfrom a point-by-point examination of the Doppler shift in the waveenergy reflected by the vibrating object under investigation 102. Theimage intensity or color is proportional to the vibration amplitudecalculated by means of the estimation method which will be describedlater herein, at each point of the object 102. That image can beinspected for modal shapes as well as abnormally high or low vibrationamplitudes, and can also be compared with reference images obtainedearlier or from well characterized analogous structures.

Referring now to FIG. 2, there is shown a block diagram of the apparatusof the present invention. As shown in that figure, the object underinvestigation 102 is vibrated by a vibration transducer 214 which ispowered by an amplifier 216. The amplifier 216 is driven by a vibrationfunction generator or vibration source 100 whose frequency and signalshape are adjustable. After the vibration of the object underinvestigation 102 has been accomplished, it is scanned by means of anultrasound transmitter/receiver 110 which is controlled by a transmitsignal generator 200 and a transmit/receive switch 202.

The transmitter/receive switch 202 functions to control the mode ofoperation of the ultrasound transmitter/receiver 110, that is, to placeit either in the transmit or receive mode. The transmit/receive switch202 causes the ultrasound transmitter/receiver 110 to be in the transmitmode while it is receiving a transmit signal from the transmit signalgenerator 202. If it is not receiving a signal from the transmit signalgenerator 202, the transmit/receive switch 202 causes the ultrasoundtransmitter/receiver 110 to be in the receive mode and it functions totransmit the Doppler echo signal received by the ultrasoundtransmitter/receiver 110 to the received signal amplifier 204.

The output from the received signal amplifier 204 is fed to both adetector 206 and to the beta estimators 220, alternative embodiments ofwhich are shown in more detail in FIGS. 4-12. The output from thedetector 206 is fed to a logrithmic amplifier 208 whose output is inturn fed to the storage and scan converter 210. The output from the betaestimators circuitry 220 is likewise fed to the storage and scanconverters circuitry 210. The output from the storage and scanconverters 210 is the vibration images which are displayed on thedisplay 113.

A timing and position control circuit 212 produces a timing and positioncontrol signal which is used to control the vibration function generator100. A synchronization signal generator 218 receives the output signalfrom the vibration function generator 100 and generates asynchronization signal S which is fed to both the beta estimatorcircuitry 220 and the storage and scan converters circuitry 210, as wellas the timing and position control circuitry 212. The timing andposition control circuitry is also connected to control the transitsignal generator 200, the transmit/receive switch 202, the receivedsignal amplifier 204, the detector 206, the logrithmic amp 208, thestorage and scan converter circuitry 212 and the beta estimatorcircuitry 220.

A conventional ultrasound B-scan color Doppler imaging instrument, suchas that manufactured by ACUSON described above, includes the equivalentof the transmit signal generator 200, the transmit/receive switch 202,the received signal amplifier 204, the detector 206, the logrithmicamplifier 208, the storage and scan converter circuitry 210, the display113 and the timing and position control circuitry 212.

Such a conventional ultrasound imaging instrument may be utilized withthe following modifications. A vibration transducer 214 is added. Asynchronization signal generator 218 generates a synchronization signalbased upon the output from the vibration function generator 100 which isfed to the beta estimator circuitry 220 and the storage and scanconverter circuitry 210. The synchronization signal consists ofinformation relating to the phase and frequency characteristics of thevibration function generator 100. Those characteristics are used in thecalculations performed by the beta estimator circuitry 220 in order toproduce the beta estimators. The estimate of beta is stored, togetherwith the conventional ultrasound received signal amplitude in thestorage and scan converters circuits 210, which outputs the informationcollected over the entire scan plane as a video signal of the vibrationimages to the display 113.

It should be noted that the output from the storage and scan convertercircuitry 210 can be two separate images, such as a conventionalultrasound image and a separate image of the variations in beta over thescan plane. Or, the output from the storage and scan converter circuitry210 may be a signal image with color overlay that can be used to displaythe beta (vibration) information overlaid on conventional gray scaleultrasound images.

FIG. 3 shows the operation of the frequency domain estimation method ofthe present invention which may be used in conjunction with thevibration parameter calculator 108 or beta estimator circuitry 220 toaffect the remote analyzing and imaging of the object underinvestigation 102. The operation of the frequency domain estimationmethod is based upon the fact that the Doppler spectrum of echoes of asinusoidally vibrating scatterer (object under investigation 102)contains discrete spectral lines weighted by Bessel functions of thefirst kind. The frequency domain estimation method is based upon the useof a new and simple relationship between the spread or variance of theDoppler spectrum of the echoes and the vibration amplitude of thescatterer.

The frequency domain estimation method for use with the remoteinspection and analyzing system produces high accuracy even underconditions when the signal-to-noise ratio of the Doppler spectrum returnfrom the scatterer is poor. In addition, deviations caused by slightnon-linearities of the vibration of the scatterer have been found tohave little contribution to the total estimation error.

Since the scattering object 102 is caused to vibrate slowly by means ofthe vibration source 100 so as to produce a wavelength much larger thanthe geometrical dimensions of the scatterer itself, the Doppler spectrumof the signals returning from essentially sinusoidally oscillatingstructures is similar to that of a pure tone frequency modulation (FM)signal. That spectrum is a Fourier series having spectral lines lyingabove and below the carrier frequency. The space in between the spectralharmonics is equal to the vibration frequency and the amplitudes ofharmonics are given by different orders of Bessel functions of the firstkind. Typically, it is desirable to determine the amplitude, phase andfrequency of the oscillating structure.

The FM spectrum is well known and, thus, its use in connection withdetermining a Doppler shift of a moving object will be discussed onlybriefly. When a moving object is illuminated with an incident laser,radio or acoustic wave, the detected back scattered signals from thatmoving object contain a frequency shift known as Doppler shift. If thescatterer is oscillating with a vibration velocity much slower than thewave speed and of a vibration frequency much less than the carrier orincident wave frequency, the spectrum of the detected scattered wavewill be similar to that of a pure tone FM process since theinstantaneous frequency of the scattered waves has a Doppler shiftproportional to the vibration velocity. The Doppler spectral moments ofthe reflected signal are usually defined as: ##EQU1## where ω.sub.δ isthe Doppler spectral spread and thus ω.sub.δ² is the variance or secondmoment, is the mean frequency shift of the Doppler spectrum (i.e., thefirst moment), and ∫(w) is the Doppler power spectrum.

Since the object under investigation 102 is vibrating, the Doppler powerspectrum can thus be written as: ##EQU2## where W_(L) is the lowfrequency vibration frequency and where the power spectrum is downshifted to a zero frequency, using, for example, quadrature detection.For this particular Bessel spectrum, the mean frequency w is 0 and thepower spectrum is therefore symmetric about a 0 frequency. The zerothmoment only has been noted by Watson in his Treatise on the Theory ofBessel Functions, Chapter 2, pp. 14-15, 31, The MacMillan Company, NewYork, N.Y. (1945) as: ##EQU3##

The second moment can be derived in a similar way to obtain the resultof: ##EQU4##

In general, all moments of the Bessel spectrum can be calculated fromthe generating function of the Bessel function given by Watson on page14, equation (1) of his Treatise. By taking the first through kthderivative of the generating function with respect to z and substitutingz=1 into the resulting expressions, all moments of the Bessel spectrumas functions of the parameters can then be calculated from the lowerorder moments by squaring and simple algebraic manipulation.

Approached from that point of view, the Bessel spectrum becomes a oneparameter function. Therefore, the second moment is a good estimator ofthe spectrum itself. Thus, the vibration amplitude can be estimated fromthe Doppler spectral spread as: ##EQU5## The above new algorithmindicates that the amplitude parameter beta can be estimated from thestandard deviation of the power spectrum.

Even utilizing the above equation, noise still presents a problem inproviding a correct parameter estimation. For example, the Dopplersignals tend to be 30-50 dB lower than the carrier in many applications.Thus, the signal-to-noise ratio for Doppler signals is usually poor.Therefore, it is necessary to remove stationary and uncorrelated noisefrom the Doppler spectral spread vibration estimator method result. Thesignal-to-noise ratio SNR can thus be represented by the ##EQU6## wherem_(O),N is the zeroth moment of the noise.

As long as the noise is stationary, the moments of the noise powerspectrum can be estimated or determined when the vibration is removed orhalted. Once the noise moments have been estimated, the noise-freevibrational Doppler spectral spread can then be determined from thenoisy signal as: ##EQU7## Even in some applications in which thevibration is inherent and cannot be controlled externally, the noisecompensation can be performed by estimating the signal-to-noise ratio aswell as the Doppler spectral spread of the noise from the finite bandwith white noise. Even if the noise is not white, the noise compensationcan still be provided as long as the noise power and noise spectralspread can be estimated by statistical techniques.

The method for applying the frequency domain estimation system is shownin FIG. 3. First, the external vibration source 100 is turned off atstep 300. Then, the spectral moments estimation for noise is performedat step 302. This is done using conventional methods such as taking theFourier transform of the noise and deriving the appropriate integralsfrom the transform, or by taking the appropriately weighted derivativeof the time domain auto-correlation function. The result of the spectralmoments estimation for noise at step 302 is the signals representing thevariance or second moment of the Doppler spectral spread, 107_(W),²_(N), and the zero moment of the noise, M_(O),N. Both of those twosignals are fed to step 308.

After the spectral moments estimation for the noise in the Dopplerspectral spread has been completed at step 302 the external vibrationsource is turned on at step 304 and then the spectral moments estimationfor the signal plus noise is produced at step 306 by conventionaltechniques. The result of the spectral moments estimation for the signalplus the noise at step 306 is the variance of the Doppler spectralspread ω.sub.δ² and the zeroth moment of the Doppler spectral spreadM_(O). Both of those signals are fed to the vibration estimation step308 which utilizes equation (8) to produce the vibration estimation ofthe amplitude which is output by the vibration parameter calculator 108.

As readily apparent to those of ordinary skill in the art, each of thesteps 300, 302, 304, 306 and 308 shown in FIG. 3 can be implemented bymeans of a computer such as vibration parameter calculator 108, which,in addition to controlling the operation of the vibration source 100,functions to calculate the spectral moments for both the noise andsignal plus noise Doppler spectra, as well as utilizing the results ofthose calculations to produce the vibration estimation result and thusthe vibration images 112.

FIG. 4 shows a schematic block diagram of the time domain estimationsystem which may be used in connection with the vibration source 100,the ultrasound transmitter/ receiver 110 and vibration parametercalculator 108 of FIG. 1 to perform non-destructive testing,sonoelasticity imaging or other remote monitoring and analysis ofstructures and soft tissue, as previously described. That isaccomplished by estimating the amplitude and frequency of a relativelylow frequency vibration of a target which has been interrogated with arelatively high frequency wave.

The method and system shown in FIG. 4 is based upon the Doppler shiftgenerated by the vibrating target, which produces a frequency modulatedecho signal. The system of FIG. 4 uses only a small fraction of the lowfrequency vibration cycle of the object under investigation 102 toobtain the estimated parameters. Therefore, real-time imaging of thevibration of the object under investigation 102 can readily be made.

The system of FIG. 4 includes methods which complement each other tocover a wide variety of the estimated parameters and different sampling,scanning and imaging criteria which are confronted in practice. Themethodology implemented in FIG. 4 produces good noise performance andlow sensitivity to non-linearities of the vibration which may be presentin non-ideal conditions of operation.

Although the pulsed Doppler technique has been successfully applied forblood flow detection for over 15 years, all of the research towardsutilizing Doppler spectral parameter estimations involving time andfrequency domain processing have been oriented towards steady and slowlyvarying or pulsatile blood flow. Such estimations are not suitable forthe applications of vibration amplitude detection using Dopplerultrasound.

Vibration images are also desirable for use in sonoelasticity imagingfor detecting a hard tumor surrounded by relatively soft tissues. Theprinciple of sonoelasticity imaging involves the vibration of the tissueby an external source which generates a "low" frequency compared to theinvestigating wave, such as the vibration source 100. Regions ofabnormal elasticity produce abnormal vibration amplitudes.

Since the Doppler spectrum received from a vibrating target is symmetricabout the center frequency, the mean frequency is zero. Therefore,conventional time domain and frequency domain velocity and "turbulence"estimators are not appropriate for detecting vibration such as thatdisclosed in U.S. Pat. No. 4,853,904, to Pesque, and thus estimatorsbased upon time domain processing must be used for sonoelasticityimaging and other applications where varying types of propagating waves,for example, ultrasound, laser, microwave and radio wave, are utilizedto detect oscillating structures. See also "Real-Time Two-DimensionalDoppler Flow Mapping Using Auto-Correlation", Acoustical Imaging, M.Kaveh et al., ed., Volume 13, Plenum Press, pp. 447-460 (1984).

The estimating methods disclosed herein utilize only very few samples ofthe Doppler signal and the resulting estimation has only a weakconnection with the phase of low frequency vibration. Thus, the presentinvention achieves an asynchronous real-time two-dimensional"sonoelasticity imaging" or vibration imaging system which overcomes theshortcomings of the prior art.

The problem of estimating the vibrational parameter underlies severalother applications aside from that of "sonoelasticity imaging". Forexample, other areas of application include remote sensing, radar,sonar, acoustics and laser calibration of sound fields. In the past,since the time domain waveform is complicated, frequency domaintechniques were the primary object of efforts to analyze the vibrationalparameters. A new and simple solution to the use of frequency domainmethods has been discussed in connection with FIG. 3.

Since known techniques typically require a long sequence of Dopplersignal to be analyzed in order to avoid frequency aliasing and toachieve noise reduction in extracting low-magnitude spectral components,they are not well suited to making real-time images. However, the timedomain processing system of the present invention requires less datathan frequency domain approaches, including that disclosed herein, toachieve the same estimation and is therefore more suitable for real-timeand/or imaging applications.

For the simplified case of a sinusoidally vibrating target, the Dopplerreturned signal can be represented by a pure tone frequency modulation(FM) process such that the receipt signal can be written as

    s.sub.r (t)=A cos (ω.sub.O t+βsin (ω.sub.L t+φ)) (10)

"`Sonoelasticity` images derived from ultrasound signals in mechanicallyvibrated tissues", R. M. Lerner, S. R. Huang, Kevin J. Parker,Ultrasound in Medicine & Biology, Vol. 16, No. 3, pp. 231-239, 1990.Since the modulation index of the FM process, beta, is directly relatedto the vibrational amplitude of the velocity or displacement field, betais equal to ##EQU8## where λ_(O) is the vibration amplitude of thedisplacement field, ω_(O) is the wavelength and theta is the anglebetween the wave propagation and the vibration vectors. Thus, theestimation of the modulation index, beta, is equivalent to theestimation of the vibration amplitude of the displacement and/orvelocity fields.

If a synchronous detection is used to detect the Doppler signal, thenthe resulting two discrete quadrature signals can be represented as:

where T is the sampling period is

    I.sub.k =cos (βsin (kω.sub.L T.sub.s +φ))

(12)

    Q.sub.k =sin (βsin (kω.sub.L T.sub.s +φ))

(13)

where T₃ is the sampling period, and k the sample number: 1, 1, 3 . . .

Since the cross products of two quadrature sinusoidal signals can besimplified using trigonometric identities, those two signals can bedefined as: ##EQU9## Those two signals are referred to as "phase" and"co-phase" signals and are both related to the phase of the receivedsignal. Derivations of those two signals involve the complexmultiplications of two successive complex quadrature signals. Uponexamining those two signals closely, it is seen that they are 90°out-of-phase with different amplitude multipliers. In other words, theyare the phase-shifted and amplified versions of the vibration signals.

Therefore, to estimate the vibration amplitude, it is known that the sumand the difference of successive samples of the phase signal can bewritten in the forms ##EQU10## Thus, if the vibration frequency isknown, the vibration amplitude can be estimated directly from the aboveequations as: ##EQU11##

The above estimator is referred to as the "successive-phase estimator".Since the sum and difference of successive "phase" and "co-phase"signals are essentially the same, the same estimation can be obtainedusing the sum and difference of the two successive samples of co-phasesignals. If both the phase and co-phase signals are available, theestimation can be achieved by combining those two signals. Since bothphase and co-phase signals have contributions depending upon thesampling rate, this estimator is called the "bi-phase estimator". Itrequires an additional arc tangent operation than does thesuccessive-phase estimator; however, it is less sensitive to thevariation of the sampling rate.

Noise is reduced during the instant estimation process by the crossmultiplications of in-phase and quadrature-phase signals (I_(k) andQ_(k)) in order to derive the phase and co-phase signals as shown inequations (12) and (13). By applying the same technique, the crossproducts of the phase and co-phase signals can be used to achieveadditional noise reduction in the following form: ##EQU12## Thatequation is referred to as the 1-shift cross-phase estimator which caneasily be generalized to n-shifts for flexible signal processing.

In all of the foregoing estimations, the vibration frequency of thevibration source 100 must be known before the vibration amplitude can beestimated. In some applications, the vibration of the object underinvestigation 102 is externally forced, such as shown in FIG. 1, and thevibration frequency can then be accurately and precisely tracked using afrequency meter 418 as shown in FIG. 4. Such is the case when conductingsonoelasticity imaging and laser calibration and measurement of soundfields. In other applications, in which internal vibrations apply, forexample, heart valve vibration, the frequency of vibration is unknownand that important parameter needs to be estimated.

By taking the ratio of the difference of the phase and the sum of theco-phase signals, the "phase-ratio estimator" or vibration frequency canbe shown to be: ##EQU13##

All of the algorithms discussed above use a fixed number of samples toestimate the parameters. Thus, the only way in which noise performancecan be improved is by averaging the number of samples over the estimatedparameters. However, the noise may be amplified in the process ofperforming those calculations, therefore, the performance improvement issomewhat limited. Since, for a small modulation index, noise causes moreserious problems in estimations, another estimation method whichperforms an averaging operation over the raw data has been developed.

It is well known that the spectrum of the pure tone FM signal is aseries of Bessel functions. If x(t) represents the complex quadraturesignal,

    x(t)=cos (βsin (ω.sub.L t+φ))+i sin (βsin (ω.sub.L t+φ))                                  (21)

the desired vibration amplitude can be estimated as ##EQU14## when theestimated parameter beta is small. This is referred to as the"auto-correlation estimator" for vibration amplitude.

In practice, the auto-correlation function is approximated by a finiteseries: ##EQU15## where x_(k) is the discrete complex quadrature signalI_(k) +iQ_(k). The parameter N can be varied according to realconditions, for example, the sampling rate, noise level andsynchronization. The longer the sequence is used to approximate theauto-correlation, the better the estimate becomes. In practice, four toten samples can be used to produce a good approximation.

If the RF sampling rate is sufficiently high, the vibration estimationcan be achieved as has been previously described. In the case which thesampling frequency is not sufficiently high or the only real RF signal(either sine or cosine or a phase-shifted sine component) is available,the vibration can still be estimated after correction of the sine and/orcosine factors and the auto-correlation can be expressed as:

    R(τ)-1/2(2-β.sup.2) cos (W.sub.O π)

This equivalence of signal processing in both the baseband and RFdomains provides a high degree of freedom and flexibility in the designof system architecture, electronic circuitry and signal processing.

The vibration amplitude can also be estimated using conventional Dopplerestimators. Since the modulated signal is sinusoidally vibrating, theoutput of a conventional mean Doppler frequency estimator is also asinusoidal function. The frequency of this sinusoid is the vibrationfrequency and the amplitude is proportional to the modulation index orvibration amplitude. Therefore, one can also upgrade a conventionalDoppler estimator with some modifications.

But, due to the nature of a truncated sequence used in conventionalDoppler estimators, large bias or variance of the resulting estimates ofthe sinusoidal amplitude are often encountered. Thus, to reduce the biasand variance, the estimation time can be made longer than that of theestimation techniques discussed above. Nonetheless, it is stillworthwhile discussing some approaches to modify a conventional Dopplerinto vibration Doppler estimates.

The first way to find the amplitude of the sinusoid is to differentiateor to integrate the sinusoidally vibrated estimate. Let the conventionalmean Doppler frequency estimate be

    y(t)=βsin (W.sub.L t+ρ).                          (27)

Therefore, the vibration amplitude can be estimated as: ##EQU16##

That method is very general but the performance depends on how well thetime derivative or integration can be achieved. The time derivative andintegration can be either derived from the conventional Doppler estimateor directly derived from the RF signal. For direct RF estimation, thattranslates to the use of displacement, velocity, and/or accelerationestimators. For example, since the time shift of the RF correlation peakbetween pulses is proportional to the displacement, and velocity can bederived from the amount of time shift of the RF correlation peak in afew successive pulses, (as per U.S. Pat. No. 4,853,904, to Pesque),those two signals can be combined to estimate the vibration amplitude.

Another approach is to use spectral estimation techniques to estimatethe power or amplitude of a truncated sinusoid. Among those spectralestimation techniques, the AR (autoregressive) and ARMA (autoregressiveand moving average) models suffer from the stochastic nature of themodeling and therefore the resulting estimates are oscillating as afunction of truncation length, initial phase of the sampled sinusoid,sampling frequency, etc. Therefore, the sampling must be held at acertain fixed operating point and the vibration frequency cannot bechanged or swept during the estimation. That restricts the use of thevibration and the scanning. Some other methods employ sinusoidal modelsand might perform better. Examples are Pisarenko Harmonic Decompositionand Prony Spectral Line Estimation, as discussed in "Spectrumanalysis--a modern perspective", S. M. Kay and J. S. Marple, Proc. IEEEVol. 69, pp. 1380-1419, Nov. 1981. For all spectral estimationtechniques, the major disadvantage is the intensive computation involvedin the least square minimization.

Another way to adapt conventional Doppler techniques is to utilize thenew discovery by the present inventors that the vibration amplitude isproportional to the spectral variance of the Doppler signal. Someconventional methods for estimating "turbulence" or variance have beenapplied to blood flow. C. Kasai et al., "Real-Time Two-Dimensional BloodFlow Imaging Using an Autocorrelation Technique", IEEE Trans. SonicsUltrason., Vol. Su-32, pp. 458-463, May 1985. In the presence of avibrating target, these estimates would oscillate, but could be adaptedwith synchronization to the vibration source, as well as by averagingtechniques. An example of the application of the teachings of theinstant invention to improving the conventional methods of estimatingare shown in FIGS. 13a and 13b which are discussed later herein.

FIG. 4 shows the time domain processing system embodiment for use withthe present invention. As shown in that figure, an oscillator 400 isused to control the frequency of oscillation of the transmitter/receiver110. The output from the transmitter/receiver 110 is fed to a firstmultiplier 404 which also receives the output signal from the oscillator400. The oscillator 400 also applies its output signal to a 90° shifter402, which shifts the phase of the output from the oscillator 400 by 90°and then applies it to a second multiplier 406. As shown in FIG. 4, theoutput from the transmitter/receiver 110 is also applied to the secondmultiplier 406.

The output from each of the respective multipliers 404 and 406 is fed torespective low pass filters 408 and 410 which function to eliminate theunwanted double frequency signals. The output from the low pass filter408 is the first quadrature signal I_(k). The output from the second lowpass filter 410 is the second quadrature signal Q_(k). The outputs fromboth of the low pass filters 408, 410 are provided to the input of thephase and co-phase estimation block 412.

The output from the phase and co-phase estimation block 412 is fed tothe input of the vibration frequency estimation block 414, whosecircuitry is shown in FIG. 11. It is also fed to the input of thevibration amplitude estimation block 416 which, in conjunction with thevibration parameter calculator 108, functions to produce the desiredvibration amplitude signal beta.

The vibration amplitude estimation block 416 also receives as an inputthe output signals from the low pass filters 408, 410 and a vibrationfrequency signal, which will be described later. The vibration amplitudeestimation block 416 solves equation numbers 6, 8, 18, 19, 22 and 24 andin order to produce the desired vibration amplitude estimation.

When the system of the present invention is operated in an externalvibration mode, the output from the vibration source 100, in addition tobeing provided to the pulse ultrasound imaging device 104 and being usedto vibrate the object under investigation 102, it is fed to a frequencymeter 418 which produces a signal representative of the vibrationfrequency. When switch 420 is in the external vibration mode setting, asis shown in FIG. 4, that frequency signal is provided as the fourthinput to the vibration amplitude estimation block 416. When the switch420 is in the internal vibration mode, the output from the vibrationfrequency estimation block 414 is provided as the frequency signal tothe vibration amplitude estimation block 416.

FIGS. 5-10 show the various circuitry which may be utilized eithersingly, or in combination with the circuitry shown in those figures, forproviding the phase and co-phase estimations.

In FIG. 5, I_(k) and Q_(k), the quadrature components, are input and thecircuit computes the phase and co-phase parameters φ_(k), φ_(K) ¹. InFIG. 6, the phase and co-phase signals are combined with delayedversions of those signals to form the intermediate parameters A, B, C &D.

FIG. 7 shows the use of the intermediate parameters A & B, combined withthe scale factors W₁ and W₂, to form the successive phase estimate.

FIG. 8 shows the use of the intermediate parameters C and D, togetherwith the scale factors, W₃ and W₄, to product the successive co-phaseestimate.

FIG. 9 shows the use of the phase and co-phase signals, combined withthe scale factors W₅ and W₆, to produce the bi-phase estimate.

FIG. 10 shows the combination of the phase and co-phase and delayedversions combined with the scale factor W₇, to produce the 1 shiftestimator.

FIG. 11 shows the combination of the phase difference and co-phasedifference signals, together with the scale factor W₈, to produce thevibration frequency estimate.

FIG. 12 shows the use of the quadrature components I_(k) and Q_(k) in anauto-correlation process. When combined with the scale factor W₉, theoutput is the correlation estimate.

These, and their obvious modifications, are used to cover a wide rangeof beta and signal to noise ratios. The output of FIG. 3 is useful for awide range of beta (10⁻⁴ to 10⁴) and high SNR (up to 0 dB or more). Thecorrelation estimate of FIG. 7, is useful in cases when beta<20. Thephase estimators, FIGS. 5-10 are useful in the range of 0.03-3. Thesuccessive phase estimator is useful for 0.1<beta and for large beta.Each estimate can be used or not used depending on a prior knowledge ofthe range of beta (often maximum at the applied vibration source),and/or by "polling" the results to utilize the most stable output in aspecific region of interest.

FIG. 5 shows the operation of the phase and co-phase signals estimationsystem which may be utilized in connection with the phase and co-phaseestimation block 412.

As shown in FIG. 5, the quadrature signals I_(k) and Q_(k) are fed todelay circuits 500, 502, as well as to two multiplying circuits 504,508. The outputs from the respective delay circuits 500, 502 are fed totwo other multipliers 506, 510, as are the signals I_(k) and Q_(k),respectively. Additionally, the outputs from the two respective delaycircuits 500, 502 are fed to the two multiplier circuits 508 and 504,respectively.

The output from the multiplier circuits 504-510 are fed respectively tothe summing circuits 512, 516; 514, 518; 516, 512; and 518, 514. Theoutputs from the two adding circuits 512 and 514 are fed to anarctangent calculating circuit 520 which produces one of the outputsignals provided to both the vibration amplitude estimation block 416and the vibration frequency estimation block 414. In a similar manner,the outputs from the two remaining adder circuits 516 and 518 are fed toa similar arctangent calculator circuit 522 to produce the second outputfrom the phase and co-phase estimation block 412.

FIGS. 6-8 show additional circuitry for refining the output signalsφ_(k) and φ'_(k) produced by the phase and co-phase estimation block 412using, for example, the circuitry of FIG. 5. FIG. 6 shows a blockdiagram of additional phase processing circuitry.

The two signals output from the phase and co-phase estimation circuit412 are fed to respective delay circuits 600, 602 as well as torespective adder circuits 604, 606 and 608, 610. In addition, the outputsignal produced by the delay 600 is fed to a first adder 604 and theoutput produced by the delay circuit 602 is fed to the first adder ofthe φ'_(k) signal 608.

The outputs from the respective adder circuits 604, 606, 608 and 610 arefed respectively to squaring circuits 612, 614, 616 and 618. Thosesquaring circuits respectively produce signals A-D which are utilized bythe circuits of FIGS. 7 and 8.

FIG. 7 shows the successive-phase estimator circuitry which utilizes thesignals A and B produced by the circuitry of FIG. 6 which are fedrespectively to two multiplier circuits 700 and 702. Each of thosemultiplier circuits also receives as an input the signals w₁ and w₂,respectively, which represent scale factors, "constants" or "gains". Theoutputs from each of the multiplier circuits 700, 702 are added by theadder circuit 704 whose output is fed to a square root circuit 706. Theoutput of the square root circuit 706 is the successive phase estimate.

FIG. 8 shows the circuitry which can be used to produce the successiveco-phase estimator component, using the signals C and D from FIG. 6.Those signals are fed respectively to two multiplier circuits 800 and802 which are also supplied with two scale factors signals w₃ and w₄,respectively. The outputs from the two multiplier circuits 800, 802 areadded by the adder circuit 804 whose output is provided to a square rootgenerator 806 which produces the successive co-phase estimate signal.

Instead of utilizing the circuitry of FIGS. 6-8 to further refine theoutput signals from the phase and co-phase estimation circuitry 412, thecircuitry of FIG. 9, which implements the bi-phase estimator method, maybe utilized. In that case, the two signals output from the phase andsuccessive co-phase estimation circuit 412 are fed respectively to twomultiplier circuits 900, 902. Those multiplier circuits are also fedwith two scale factors signals w₅ and w₆, respectively. The outputs fromthe two multiplier circuits 900 and 902 are added by the adder circuit904- The output from the adder circuit 904 is fed to a square rootgenerator 906 whose output signal represents the bi-phase estimate.

FIG. 10 shows the circuitry for producing a 1-shift cross-phase valueestimator for use with the circuitry of FIGS. 5-9. The signals k and fedto first and second multipliers 1000 and 1002. The outputs from thosemultipliers 1000, 1002 are fed to an adder circuit 1004 whose output isfed to a third multiplier circuit 1006. The third multiplier circuit1006 is also fed with a signal w₇. The output from the multiplier 1006is fed to a square root generator 1008 which produces the 1-shiftcross-phase signal at its output.

FIG. 11 shows the circuitry for implementing the phase-ratio estimatorfor vibration frequency which utilizes as its input the signals φ_(k)-and φ'_(k) +from the circuitry of FIG. 6. The circuitry of FIG. 11 maybe utilized as the vibration frequency estimation circuitry for element414 of FIG. 4.

The two signals previously described are fed to a divider 1100. Theoutput from the divider 1100 is fed to a square root generator 1102which takes the square root of the results from the divider 1100 andapplies it to an arctangent generator 1104. The output from thearctangent generator 1104 is fed to a multiplier 1106 which alsoreceives the scale factor signal w₈. The output from the multipliercircuit 1106 represents an estimation of the internal vibration modefrequency and is fed to the vibration amplitude estimation circuitry 416of FIG. 4.

FIG. 12 shows an auto-correlation estimator which may be used inconnection with the circuitry of the present invention. FIG. 12 utilizesthe number of ultrasound pulses, N, in order to estimate theauto-correlation and quasi-auto-correlation which can be varied to givethe desired best combination of signal performance and frame rate. Thechoice of N also depends upon the vibration frequency of the appliedexternal excitation.

The signal x_(k) is applied to a delay and conjugate circuit 1200 aswell as to a first multiplier 1202. The output from the delay andconjugate circuit 1200 is also applied to the multiplier 1202. Theoutput from the multiplier 1202 is applied to an integrator andmagnitude circuit 1204 whose output is fed to an adder circuit 1206. Thesignal unity or 1 is also fed to the adder 1206.

The output from the adder 1206 is fed to a second multiplier 1208 whichalso receives the scale factor signal w₉. The output from the multipliercircuit 1208 is fed to a square root generator 1210 which produces thedesired correlation estimate signal.

FIGS. 13A and 13B show, respectively, how the teachings of the presentinvention can be applied to produce approximate estimations of vibrationamplitude signals utilizing conventional velocity estimators (FIG. 13A)or conventional variance estimators (FIG. 13B).

Referring to FIG. 13A, a conventional velocity estimator 1300 is shown,which may, for example, be that of Kasai et al. or Pesque, both of whichhave been discussed previously herein. The output y(t) produced by theconventional velocity estimator 1300 is fed to both an integrator ordifferentiator 1302 and an adder 1306. The output from the integrator ordifferentiator 1302 is multiplied by a scale factor w₁₀. The output fromthe multiplier is added in the adder 1306 to the output from theconventional velocity estimator 1300. A square root generator 1308produces the square root of the sum produced by the adder 1306, which isan approximate estimation of the vibration amplitude. It should beemphasized that the output from the square root generator 1308, whilebeing more accurate than the output y(t) from the conventional velocityestimator 1300, is not nearly as accurate or precise as the output fromthe estimator systems disclosed herein.

FIG. 13B shows how the output from a conventional variance estimator1310, such as that disclosed by Kasai et al., can be modified in orderto produce a more accurate, although approximate estimation of vibrationamplitude. The output from the conventional variance estimator 1310,v(t) is fed to an average and synchronize circuit 1312 and is thenmultiplied in a multiplier 1314 by a scale factor w₁₁. The output fromthe multiplier 1314 is the approximate estimation of the vibrationamplitude.

As will be obvious to those of ordinary skill in the art, othervariations of the present invention utilizing the time domain estimatormethodology disclosed herein may be realized utilizing radio frequencysignals (since there is a one-to-one correspondence between the basebandand RF signal processing), a combination of velocity, displacement,and/or acceleration estimators, or using spectral estimation techniquesafter those techniques have been adapted for estimating a truncatedsinusoid on the phase or frequency estimates.

For example, a filter to eliminate unwanted signals, such as those fromstationary targets, or those moving with slow steady (not vibrating)velocity, may be added. These are commonly referred to as "wallfilters", "fixed echo suppressors" or "moving target indicators".

Additionally, commonly employed circuits could be added to suppress andnot display estimation results when the received echoes are too weak ortoo strong. These are commonly referred to as "discriminator circuits","squelch circuits" or "suppressor circuits".

Although only a preferred embodiment is specifically illustrated anddescribed herein, it will be appreciated that many modifications andvariations of the present invention are possible in light of the aboveteachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. The system using Doppler modulation parameters inreal-time for determining the vibration amplitude of an object,comprising:means for causing said object to vibrate; means forgenerating one of pulsed electromagnetic and acoustic radiation and forimpinging said pulsed radiation onto said object; means for receivingpulsed radiation Doppler shifted signals reflected by said object afterimpingement of said pulsed radiation on said object; and means foranalyzing said Doppler shifted signals to derive the vibration amplitudeof said object.
 2. The system of claim 1, wherein said means for causingsaid object under investigation to vibrate is a low frequency vibrationsource having a frequency of between approximately one one-millionth toone one-thousandth of the interrogating frequency.
 3. The system ofclaim 1, wherein said means for analyzing said Doppler shifted signalscomprises a frequency domain estimation system.
 4. The system of claim3, wherein said frequency domain estimation system comprises:means forturning on and off said means for causing said object to vibrate; meansfor calculating the spectral moments of noise when said object is notvibrating;
 5. The system of claim 1, wherein said means for analyzingsaid Doppler shifted signals comprises a time domain estimationsystem.means for calculating the spectral moments of said Dopplershifted signals and noise when said object is being vibrated; and meansfor receiving said calculation of the spectral moments of noise and saidcalculation of the spectral moments of said Doppler shifted signals andnoise and for calculating said vibration amplitude of said object. 6.The system of claim 5, wherein said time domain-estimation systemcomprises:means for generating quadrature phase signals representativeof said Doppler shifted signals; means for deriving the phase andco-phase value signals of said quadrature phase signals; and means forreceiving said quadrature phase, phase and co-phase signals and forgenerating said amplitude of vibration of said object based upon saidreceived signals.
 7. The system of claim 6, wherein said means forreceiving said quadrature, phase and co-phase signals also receives asignal generated by said means for causing said object to vibrate foruse in generating said amplitude of vibration of said object.
 8. Thesystem of claim 6, further including means for generating a vibrationfrequency signal corresponding to the vibration frequency of said objectand for providing said vibration frequency signal to said means forreceiving said quadrature, phase and co-phase signals.
 9. The system ofclaim 6, wherein said means for receiving said quadrature, phase andco-phase signals comprises means for phase processing said quadrature,phase and co-phase signals.
 10. The system of claim 6, wherein saidmeans for receiving said quadrature, phase and co-phase signalscomprises means for successively processing said phase signals.
 11. Thesystem of claim 6, wherein said means for receiving said quadrature,phase and co-phase signals comprises means for successively processingsaid co-phase signals.
 12. The system of claim 6, wherein said means forreceiving said quadrature, phase and co-phase signals comprises meansfor processing bi-phase signals.
 13. The system of claim 6, wherein saidmeans for receiving said quadrature, phase and co-phase signalscomprises means for n-shift cross-phase processing said quadrature,phase and co-phase signals.
 14. The system of claim 9, wherein saidmeans for generating a vibration frequency signal corresponding to saidvibration frequency comprises means for generating a phase ratio usingsaid phase and co-phase signals.
 15. The system of claim 1, wherein saidmeans for generating one of pulsed electromagnetic and acousticradiation scans said object.
 16. The system of claim 1, wherein saidmeans for causing said object to vibrate is a source of one of pure toneand broad band signals.
 17. A method for the real-time calculation ofthe vibration amplitude of an object using Doppler shifted signalsreflected from the object, comprising the steps of:vibrating said objectusing a source of vibration; impinging one of pulsed electromagnetic andacoustic radiation onto said object; receiving Doppler shifted signalsreflected from said surface object; and analyzing said Doppler shiftedsignals to derive the vibration amplitude of said object.
 18. The methodof claim 17, further including the step of scanning said object withsaid impinging pulsed radiation.
 19. The method of claim 18, whereinsaid step of analyzing is accomplished using a frequency domainestimator system.
 20. The method of claim 17, wherein said source ofvibration has a frequency of approximately 1-1000 Hertz.
 21. The methodof claim 17, wherein said step of analyzing is accomplished using a timedomain estimator system.
 22. The method of claim 17, wherein said stepof analyzing comprises the steps of:turning off said source ofvibration; estimating the spectral moments of noise; turning on saidsource of vibration; calculating the spectral moments of the noise andof said Doppler shifted signals; estimating the spectral moments of thenoise and of said Doppler shifted signals; and calculating the vibrationamplitude using said calculated spectral moments of noise and saidDoppler shifted signals and said calculated spectral moments of noise.23. The method of claim 17, wherein said step of analyzing comprises thesteps of:generating quadrature phase signals representative of saidDoppler shifted signals; deriving the phase and co-phase value signalsof said quadrature phase signals; and calculating the vibrationamplitude of said object based upon said quadrature phase, phase andco-phase value signals.
 24. The method of claim 17, wherein said sourceof vibration produces one of pure tone and broad band signals.
 25. Thesystem for the real-time estimation of the vibration amplitude of anobject using Doppler shifted signals reflected by said object,comprising:means for generating vibration signals for causing vibrationof said object; means connected to said means for generating vibrationsignals for generating synchronization signals synchronized to saidvibration signals; a Doppler scan imaging device connected to receivesaid synchronization signals and to control said means for generatingvibration signals; and means for analyzing Doppler shifted signals toderive the vibration amplitude of said object, said means for analyzingbeing connected to both said means for generating synchronizationsignals and said Doppler scan imaging device.
 26. The system of claim25, wherein said means for analyzing said Doppler shifted signalscomprises a frequency domain estimation system.
 27. The system of claim25, wherein said means for analyzing said Doppler shifted signalscomprises a time domain estimation system.
 28. The system of claim 25,wherein said means for generating vibration signals generates one ofpure tone and broad band signals.
 29. The system using Dopplermodulation parameters in real-time for determining the vibrationamplitude of an object, comprising:means for causing said object tovibrate; means for generating coherent radiation and for impinging saidcoherent radiation onto said object; means for receiving coherentradiation Doppler shifted signals reflected by said object afterimpingement of said coherent radiation on said object; means foranalyzing said Doppler shifted signals to derive the vibration amplitudeof said object; and said means for analyzing said Doppler shiftedsignals comprising a time domain estimation system comprising:means forgenerating quadrature phase signals representative of said Dopplershifted signals; means for deriving the phase signals; and means forreceiving said quadrature phase, phase and co-phase signals and forgenerating said amplitude of vibration of said object based upon saidreceived signals.
 30. The system of claim 29, wherein said means forreceiving said quadrature, phase and co-phase signals also receives asignal generated by said means for causing said object to vibrate foruse in generating said amplitude of vibration of said object.
 31. Thesystem of claim 29, further including means for generating a vibrationfrequency signal corresponding to the vibration frequency of said objectand for providing said vibration frequency signal to said means forreceiving said quadrature phase and co-phase signals.
 32. The system ofclaim 29, wherein said means for receiving said quadrature, phase andco-phase signals comprises means for phase processing said quadrature,phase and co-phase signals.
 33. The system of claim 32, wherein saidmeans for generating a vibration frequency signal corresponding to saidvibration frequency comprises means for generating a phase ratio usingsaid phase and co-phase signals.
 34. The system of claim 29, whereinsaid means for receiving said quadrature, phase and co-phase signalscomprises means for successively processing said phase signals.
 35. Thesystem of claim 29, wherein said means for receiving said quadrature,phase and co-phase signals comprises means for successively processingsaid co-phase signals.
 36. The system of claim 29, wherein said meansfor receiving said quadrature, phase and co-phase signals comprisesmeans for processing bi-phase signals.
 37. The system of claim 29,wherein said means for receiving said quadrature, phase and co-phasesignals comprises means for n-shift cross-phase processing saidquadrature, phase and co-phase signals.
 38. A method for the real-timecalculation of the vibration amplitude of an object using Dopplershifted signals reflected from the object, comprising the stepsof:vibrating said object using a source of vibration; impinging coherentradiation onto said object; receiving Doppler shifted signals reflectedfrom said object; and analyzing said Doppler shifted signals to derivethe vibration amplitude of said object; and wherein said step ofanalyzing said Doppler shifted signals comprises the steps of:turningoff said source of vibration; estimating the spectral moments of noise;turning on said source of vibration; calculating the spectral moments ofthe noise and of said Doppler shifted signals; estimating the spectralmoments of the noise and of said Doppler shifted signals; andcalculating the vibration amplitude using said calculated spectralmoments of noise and said Doppler shifted signals and said calculatedspectral moments of noise.